[SPARK-50384] Update docs with YuniKorn 1.6.0 and first-time installation guide

### What changes were proposed in this pull request?

This PR updates docs for first-time YuniKorn installation in cluster, in case reader has not yet installed YuniKorn previously. Also added docs link to

### Why are the changes needed?

If the reader has not yet installed YuniKorn charts locally before, they would need to add the repo first.

### Does this PR introduce _any_ user-facing change?

Docs only

### How was this patch tested?

Pass the CIs

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #147 from jiangzho/doc.

Lead-authored-by: zhou-jiang <[email protected]>
Co-authored-by: Zhou JIANG <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: jiangzho

README.md

@@ -86,8 +86,19 @@ sparkcluster.spark.apache.org "prod" deleted
 
 ## Run Spark Pi App on Apache YuniKorn scheduler
 
+If you have not yet done so, follow [YuniKorn docs](https://yunikorn.apache.org/docs/#install) to install the latest version: 
+
+```bash
+$ helm repo add yunikorn https://apache.github.io/yunikorn-release
+
+$ helm repo update
+
+$ helm install yunikorn yunikorn/yunikorn --namespace yunikorn --version 1.6.0 --create-namespace --set embedAdmissionController=false
+```
+
+Submit a Spark app to YuniKorn enabled cluster:
+
 ```bash
-$ helm install yunikorn yunikorn/yunikorn --namespace yunikorn --version 1.5.2 --create-namespace --set embedAdmissionController=false
 
 $ kubectl apply -f examples/pi-on-yunikorn.yaml